Method for producing nitric acid

ABSTRACT

A process for producing nitric acid of a concentration in the range of 68 to 76% by weight, using the mono-pressure or the dual-pressure process in which the ammonia feedstock is combusted with the aid of compressed process air. The water vapour content of the process air used for combustion and/or stripping and imported from outside the system, is reduced in this process.

BACKGROUND OF THE INVENTION

The present invention relates to a process for the production of nitricacid. The described process for the production of nitric acid isespecially based on the mono-pressure or the dual-pressure process inwhich the ammonia feedstock is combusted with the aid of compressedprocess air and the nitrous gas formed during combustion is at leastpartly absorbed by water, thus forming nitric acid, and the non-absorbedtail gas is expanded from the second pressure to ambient pressure in atail gas expansion turbine for generating compression energy.

The first step in the production of nitric acid is the reaction ofammonia NH₃ with air yielding nitrogen oxide NO:4NH₃+5O₂→4NO+6H₂O+907.3 kJ.

The nitrogen oxide NO thus obtained is then oxidised to form nitrogendioxide NO₂:2NO+O₂→2NO₂+113.1 kJ.

Finally, the nitrogen dioxide NO₂ thus obtained is absorbed in wateryielding nitric acid:4NO₂+O₂+2H₂O→4 HNO₃+256.9./.390.3 kJ.

In order to ensure that the water absorbs the maximum possible portionof the nitrogen dioxide NO₂, the absorption is carried out at elevatedpressure. The absorption pressure preferably ranges between 4 and 14bar.

The oxygen required for the reaction of the ammonia feedstock issupplied in the form of atmospheric oxygen. The process air iscompressed to a pressure which suits both the oxidation reaction and theabsorption reaction.

The energy required for compressing the air is generated, on the onehand, by expanding the tail gas leaving the absorption unit to ambientpressure and, on the other hand, by utilising the heat dissipated in thereactions.

The designs of the different types of nitric-acid production plants arebased on the specific requirements of the individual location.

Single-line nitric-acid plants are usually designed and rated forcapacities between 100 and 1000 tonnes of nitric acid per day. If thesize of the reaction unit is doubled, a single line can yield a dailyproduction of up to 2000 tonnes.

Should a low daily production be required or should the energy pricesprevailing at the location be comparatively low, the nitric-acidproduction plant will be designed on the basis of the high-pressuremono-pressure process. In this process, the pressure applied in theammonia combustion and nitrogen oxide absorption units will be aboutequal, i.e. approx. 10 bar.

Should large rated capacities and/or higher acid concentrations berequired, it will be more economical to base the design of thenitric-acid production plant on the dual-pressure process.

In the dual-pressure process, the ammonia is combusted at a firstpressure which is lower than the absorption pressure. The nitrous gasesformed during the combustion are cooled and then compressed to the levelof the second pressure, i.e. the absorption pressure.

The formerly constructed plants using normal-pressure combustion andmedium-pressure absorption have nowadays been replaced by plants usingthe more cost-effective mono-pressure or dual-pressure process. Thenitric acid obtained is also referred to as sub-azeotropic nitric acidbecause, if such acid is distilled in a downstream distillation unit,the maximum nitric acid concentration that can be achieved will be 68%due to the formation of an azeotrope. The relevant literature describesa great variety of processes conceived to overcome this limit.

The end users of nitric acid, however, frequently wish to have a nitricacid of a concentration which is only slightly above such 68%, forinstance when using nitric acid in the production of adipic acid,caprolactam, toluene diisocyanate or other substances that are nitratedwith nitric acid. It is hence a long-standing need of industry to haveavailable a cost-effective process for the production of nitric acid ofa concentration between 68 and 76%.

The aim of the present invention, therefore, is to improve theconventional mono-pressure and dual-pressure processes for theproduction of sub-azeotropic nitric acid by simple and cost-effectivemeans in order to permit the production of that nitric acid at aconcentration of up to 76%.

SUMMARY OF THE INVENTION

According to the present invention, this aim can be achieved by reducingthe water vapour content of the process air imported from outside thesystem so that the air is dried. The present invention is based on theidea of minimising the quantity of water entering the system. Themoisture entrained by the air enters the HNO₃ degassing column ascombustion air and stripping air and has a considerable share in theentry of water. The effect of drying the air is that less moistureenters the overall process.

According to another embodiment of the present invention, the processair which is supplied to the combustion unit is dried.

According to another embodiment of the present invention, the strippingair, also referred to as secondary air and used for stripping theproduced nitric acid to remove the dissolved NO₂ and NO, undergoesdrying.

According to another embodiment of the present invention, the strippingair, also referred to as secondary air and used for stripping theproduced nitric acid to remove the dissolved NO₂ and NO, is dried againby scrubbing it with highly concentrated nitric acid.

All before-mentioned embodiments are based on the same inventive idea:by taking into consideration the economic criteria applicable in theparticular case, the engineer who is involved in the design or revamp ofa plant has to decide the intensity of drying the air in order toachieve the desired effect and which of the air streams should be driedfor this purpose. Technical criteria to be considered in this connectionare the air water-vapour load to be expected in the place where theprocess is run, the efficiency of the process-specific combustion of NH₃with air to form NO_(x)—planned or already in operation—and the desiredconcentration of the nitric acid to be produced. Approx. 80% of theprocess air is normally used for combustion and approx. 20% forstripping, which allows an economically optimised individual drying ofthe different process air streams.

According to another embodiment of the present invention, cooling waterof a temperature between 1° C. and 20° C. is provided for drying.

According to another embodiment of the present invention, a coolant of−25° C. to 5° C. is provided for drying.

It is expedient to install the drier for drying the process air streamsdownstream of the air compressor, but this is not necessarily requiredto achieve the aim of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of the invention are illustrated by the two typical arrangementsshown in the accompanying drawings, each of which is represented in onefigure:

FIG. 1 depicts a mono-pressure process; and

FIG. 2 depicts a dual-pressure process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a mono-pressure process with NH₃ evaporator 1, NH₃ gaspre-heater 2, NH₃ gas filter 3, NH₃/air mixer 4, air filter 5, aircompressor 6, invention-specific air drier 20, NH₃ burner 7 with LaMontwaste heat boiler, tail gas heater 8, gas cooler 9, absorption tower 13,HNO₃ degassing column 14, secondary air drier 22, tail-gas pre-heater19, NO_(x) reactor 21, tail-gas expansion turbine 15, steam drum 16,steam condensation turbine 17, and condenser 18.

Liquid ammonia is supplied at approx. 16 bar abs. and approx. 25° C. andfed to NH₃ evaporator 1. The evaporation in the latter takes place atabout 14 bar abs. which corresponds to an evaporation temperature of 36°C. In NH₃ evaporator 1, which is fed with low-pressure steam, the liquidammonia evaporates almost completely at variable temperatures. Theevaporation temperature increases in relation to the rising watercontent in the evaporator. The pressure in the evaporation system can beadjusted by varying the respective levels and the amount and/or pressureof the low-pressure steam.

The evaporated ammonia passes through a demister before it is fed tosteam-heated NH₃ gas pre-heater 2 in which it is heated to 140° C. andthen to NH₃ gas filter 3 to remove all solid particles entrained, ifany.

The compressor of the turbo set—consisting of air compressor 6, tail-gasexpansion turbine 15 and steam condensation turbine 17—takes in throughair filter 5 the moist atmospheric air, which is laden with water vapour23 and required for the process, and compresses it to 12 bar abs. at atemperature of approx. 250° C.

According to the present invention, this air stream is dried, thuswithdrawing as much moisture in this example as required to attain anitric acid concentration of 76%. The air drier 20 used in this exampleis provided with an integrated air/air heat exchanger which pre-coolsthe air entering air drier 20 to approx. 20 to 40° C. Subsequently thepre-cooled air is cooled to approx. 1° C. by chilled water in anindirectly acting cooler integrated into air drier 20; the moistureentrained in the air precipitating on the cooler surfaces as a result ofthe air temperature that falls below the dew point so that the moistureis separated from the air. When the air leaves the cooler, the waterload of the air is lower than that at the time when entering the system,i.e. the air has now been dried.

The dried air is fed to the heat-absorbing side of the air/air heatexchanger integrated into air drier 20 where the dried air is reheatedto 220° C.

The dried and heated air stream that leaves air drier 20 is divided intotwo process-air streams (primary and secondary air) 24 and 25.

Process air 24 (primary air) and ammonia gas are supplied to NH₃/airmixer 4. The ammonia content in the gas mixture is kept constant atabout 10.1% by vol. using a ratio controller. In downstream NH₃ burner 7the ammonia oxidises in the presence of a Pt—Rh catalyst at atemperature of 900° C. to form nitrogen oxide. The hot combustion gasflows through the LaMont waste heat boiler which forms a constructionalunit with NH₃ burner 7 and through the tail gas heater 8 so that thereaction heat generated during oxidation to form NO and NO₂ is almostcompletely utilised for steam generation and as input energy (tail-gasexpansion turbine 15).

Gas cooler 9 serves to cool the nitrogen oxide to approx. 50° C. bymeans of circulated cooling water, which results in the condensation ofthe major part of the reaction water from the combustion and in theformation of nitric acid with a concentration of approx. 44 to 50% bywt.

An acid condensate pump (not shown in FIG. 1) sends the acid to a sievetray in absorption tower 13, said tray having a similar acidconcentration.

Process air 25 (secondary air) is cooled to approx. 60° C. to 80° C.,the heat being transferred to the tail gas from absorption tower 13. InHNO₃ degassing column 14, which is also referred to as bleaching column,process air 25 is used for degassing the crude acid, the air becomingladen with nitrous gas and then being admixed to the main gas streamupstream of the absorption unit. Prior to fulfilling this function inHNO₃ degassing column 14, said air is scrubbed with product-grade nitricacid and thus undergoes a secondary drying in invention-specificsecondary air drier 22 which in this example is a HNO₃ scrubber.

The remaining NO gas enters absorption tower 13 at a temperature ofapprox. 56° C. This tower is equipped with sieve trays. The formation ofnitric acid is achieved in a flow counter-current to NO gas and processwater which is fed to the top tray. In accordance with the equilibriumbetween NO₂ and HNO₃ the acid concentration decreases towards the top ofthe column as the NO₂ concentration diminishes. The generated reactionheat and part of the sensible heat are dissipated by the cooling watercirculated in the cooling coils that are installed on the sieve trays.Depending on the concentration of the acid, the acid is withdrawn fromthe 1^(st), 2^(nd) or 3^(rd) sieve tray (counted from the bottom) ofabsorption tower 13.

The withdrawn crude acid is piped to HNO₃ degassing column 14, which ispacked with Pall rings, and freed from physically dissolved nitrogenoxides in a flow counter-current to process air 25 (secondary air).

Part of the nitric acid leaving HNO₃ degassing column 14 isproduct-grade nitric acid, another part is used for scrubbing thesecondary air in secondary air drier 22. The nitric acid thus diluted iseither admixed to the condensate of gas cooler 9 or directly fed to asieve tray in absorption tower 13, said sieve having a similarconcentration.

The tail gas leaves the absorption unit at the head of absorption tower13. It is then heated step by step from 25° C. to approx. 350° C., firstin tail-gas pre-heater 19 in counter-current with secondary air and thenin tail gas heater 8 in counter-current with NO gas. After the catalyticremoval of nitric oxides in NO_(x) reactor 21, it is expanded intail-gas expansion turbine 15.

FIG. 2 depicts a dual-pressure process with NH₃ evaporator 1, NH₃ gaspre-heater 2, NH₃ gas filter 3, NH₃/air mixer 4, air filter 5, aircompressor 6, invention-specific air drier 20, NH₃ burner 7 with La Montwaste heat boiler, tail gas heater 8, gas cooler 9, NO compressor 10,tail gas heater 11, gas cooler 12, absorption tower 13, HNO₃ degassingcolumn 14, secondary air drier 22, tail-gas pre-heater 19, tail-gasexpansion turbine 15, steam drum 16, steam condensation turbine 17 andcondenser 18.

Liquid ammonia is supplied to NH₃ evaporator 1 at a pressure of approx.11 bar abs. and a temperature of approx. 25° C. The evaporation in NH₃evaporator 1 takes place at about 7.0 bar abs. which corresponds to anevaporation temperature of 14° C. Hot return cooling water is fed to NH₃evaporator 1 so that the liquid ammonia evaporates almost completely atvariable temperatures. The evaporation temperature rises as a functionof the rising water content in the evaporator. The pressure of theevaporation system can be adjusted by varying the respective levels andcooling water flow rates.

The evaporated ammonia passes through a demister before it reachessteam-heated NH₃ gas pre-heater 2, in which it is heated to 80° C., andthen to NH₃ gas filter 3 to remove all solid particles entrained, ifany.

The compressor of the turbo set—consisting of air compressor 6, NOcompressor 10, tail-gas expansion turbine 15 and steam condensationturbine 17—takes in through air filter 5 the moist atmospheric processair 23 laden with water vapour, which is required for the process,through air filter 5 and compresses it to 5.6 bar abs. at a temperatureof approx. 254° C.

According to the present invention, this air stream is dried,withdrawing as much moisture in this example as required to attain anitric acid concentration of 76%. The air drier 20 used in this exampleis provided with an integrated air/air heat exchanger which cools theair entering air drier 20 to approx. 20 to 40° C. Subsequently thepre-cooled air is cooled to approx. 1° C. by chilled water in anindirectly acting cooler integrated into air drier 20; the moistureentrained in the air precipitates on the cooling surfaces as a result ofthe air temperature that falls below the dew point so that the moistureis separated from the air. When the air leaves the cooler, the waterload of the air is lower than that at the time when entering the system,i.e. the air has now been dried.

The dried air is passed to the heat-absorbing side of the air/air heatexchanger integrated into air drier 20 where the dried air is reheatedto 220° C.

The dried and heated air stream that leaves air drier 20 is divided intotwo process-air streams (primary and secondary air) 24 and 25.

Process air 24 (primary air) and ammonia gas are supplied to NH₃/airmixer 4. The ammonia content in the gas mixture is kept constant atabout 10.2% by vol. using a ratio controller. In the downstream NH₃burner 7 the ammonia oxidises in the presence of a Pt—Rh catalyst at atemperature of 890° C. to form nitrogen oxide. The hot combustion gasflows through the LaMont waste heat boiler which forms a constructionalunit with NH₃ burner 7 and through the tail gas heater 8 so that thereaction heat generated during oxidation to form NO and NO₂ is almostcompletely utilised for steam generation and as input energy (tail-gasexpansion turbine 15).

Gas cooler 9 serves to cool the nitrogen oxide to approx. 50° C. bymeans of recycle cooling water, which results in the condensation of themajor part of the reaction water from the combustion unit and in theformation of nitric acid with a concentration of approx. 44 to 50% bywt. An acid condensate pump (not shown in FIG. 2) sends the acid to asieve tray in absorption tower 13, said tray having a similar acidconcentration.

The cooled combustion gas from NO compressor 10 is then furthercompressed to 11 bar resulting in a temperature increase. The heated gasis cooled to 55° C. in tail gas heater 11 and gas cooler 12, causing theformation of further nitric acid which is also sent to a sieve tray inabsorption tower 13, said tray having a similar concentration.

Process air 25 (secondary air) is cooled to approx. 60° C. to 80° C. intail gas pre-heater 19, the heat being transferred to the tail gas fromabsorption tower 13. In HNO₃ degassing column 14, which is also referredto as bleaching column, process air 25 is used for degassing the crudeacid, the air becoming laden with nitrous gas and then being admixed tothe main gas stream upstream of the absorption unit. Prior to fulfillingthis function in HNO₃ degassing column 14, said air is scrubbed withproduct-grade nitric acid and thus undergoes a secondary drying ininvention-specific secondary air drier 22, which in this example is aHNO₃ scrubber.

The remaining NO gas enters absorption tower 13 at a temperature ofapprox. 56° C. This tower is equipped with sieve trays. The formation ofnitric acid is achieved in a flow counter-current to NO gas and processwater which is fed to the top tray. In accordance with the equilibriumbetween NO₂ and HNO₃ the acid concentration decreases towards the top ofthe column as the NO₂ concentration diminishes. The generated reactionheat and part of the sensible heat are dissipated by the cooling watercirculated in cooling coils installed on the sieve trays. Depending onthe concentration of the acid, the acid is withdrawn from the 1^(st),2^(nd) or 3^(rd) sieve tray (counted from the bottom) of absorptiontower 13.

The crude acid withdrawn is fed to HNO₃ degassing column 14, which ispacked with Pall rings, and freed from physically dissolved nitrogenoxides in a flow counter-current to process air 25 (secondary air).

Part of the nitric acid leaving HNO₃ degassing column 14 isproduct-grade nitric acid, another part is used for scrubbing thesecondary air in secondary air drier 22. The nitric acid thus diluted iseither admixed to the condensate from gas cooler 9 or directly fed to asieve tray in absorption tower 13, said sieve having a similarconcentration.

The tail gas leaves the absorption unit and is then heated step by stepfrom 25° C. to approx. 350° C., first in tail-gas pre-heater 19 in aflow counter-current to secondary air and then in tail gas heaters 8 and11 in a flow counter-current to NO gas. Subsequently, it is expanded intail-gas expansion turbine 15.

LEGEND

 1 NH₃ evaporator  2 NH₃ gas pre-heater  3 NH₃ gas filter  4 NH₃/airmixer  5 Air filter  6 Air compressor  7 NH₃ burner  8 Tail gas heater 9 Gas cooler 10 NO compressor 11 Tail gas heater 12 Gas cooler 13Absorption tower 14 HNO₃ degassing column 15 Tail-gas expansion turbine16 Steam drum 17 Steam condensation turbine 18 Condenser 19 Tail-gaspre-heater 20 Air drier 21 NO_(x) reactor 22 Secondary air drier 23Process air 24 Process air (primary air) 25 Process air (secondary air)

1. Process for the production of nitric acid of a concentration ranging from 67 to 76% by wt., using the mono-pressure or the dual-pressure process in which the ammonia feedstock is combusted with the aid of compressed process air and in which the nitrous gas formed during combustion is at least partly absorbed by water, thus forming nitric acid, wherein the water vapour content of the process air which is imported from outside the system is reduced by drying; wherein a portion of the process air is used for stripping the produced nitric acid to remove the dissolved NO₂ and NO, and this portion of the process air undergoes additional drying by scrubbing it with nitric acid.
 2. Process according to claim 1, wherein the water separated in the drying section is used as process water.
 3. Process according to claim 1, wherein the process air is dried by heat exchange with chilled water of a temperature between 1° C. and 20° C.
 4. Process according to claim 1, wherein the process air is dried by way of heat exchange with coolant of a temperature of −25° C. to 5° C. 